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Calibration of Ultrasonic Testing for faults detection in stone masonry

M. Usai1, S. Carcangiu1, and G. Concu2

1 Department of Electric and Electronic Engineering

2 Department of Civil Engineering, Environmental and Architecture

University of Cagliari, Sardinia, Italy

COMSOL CONFERENCE EUROPE 2012

Excerpt from the Proceedings of the 2012 COMSOL Conference in Milan

Outline


• Introduction Non-Destructive Testing

Ultrasonic Testing

• Experimental session

• Simulations with COMSOL Multiphisycs

• Comparison between experimental and simulated data

• Results and conclusions


The Research Group in Electrical Engineering of Cagliari, with colleagues of the Department of Civil Engineering, has developed skills and knowledge in the field of signal processing in Non-Destructive-Testing-Techniques NDTs on homogeneous materials (metals) and non-homogeneous materials (building materials).

Introduction


In the field of assessment methodologies, particular importance is given to Non-Destructive-Testing-Techniques NDTs, which aspire to achieve the highest number of information about materials and structures without altering their condition.

The implementation of algorithms for signals post-processing consists in extracting information from test signals derived from the non-destructive tests.

Introduction


Among NDTs, Ultrasonic Testing exploits the transmission and reflection characteristics of mechanical waves with appropriate frequencies passing through the investigated item.

Non-Destructive Testing

WALL

RECEIVER TRANSDUCERDIGITAL

OSCILLOSCOPE

PULSER

RECEIVER

EMITTER TRANSDUCER


Elastic waves propagate in different manner through solid materials and cavities, thus enabling fault detection.

Ultrasonic Testing

For example, the figure shows how in the presence of a fault, the transmitted wave gives rise to a signal of lower amplitude.


The testing method is the Direct Transmission Technique (EN 14579 2004). In this method, the ultrasonic wave is transmitted by a transducer (Transmitter) through the test object and received by a second transducer (Receiver) on the opposite side of the structure.

Ultrasonic Testing


Acoustic field of the transducer

Due to media dissipative effect, elastic waves are strongly attenuated

the emission cone of the signal source can be less divergent as possible.

D diameter of the Transducer source λ wavelength of the wave Near field distance

4λ

DN

2

The sound field of a transducer is divided into two zones: the near field and the far field. Near field: region directly in front of the transducer where the amplitude goes through a series of maxima and minima and ends at the last maximum, at distance N from the transducer. Far field: area beyond N, where the sound field pressure gradually drops to zero. Because of the variations within the near field it can be difficult to accurately evaluate flaws using amplitude based techniques.

Ultrasonic Testing


Beam Spread and Half Angle

If D >> λ the wave is emitted with a not very divergent cone.

As λ is in inverse proportion to frequency f, it is understandable how high frequency signals enable waves to be highly directional.

Due to media dissipative effect, elastic waves are strongly attenuated

the emission cone of the signal source can be less divergent as possible.

All ultrasonic beams diverge

the beam spread is a function of wavelength

D

2.1sin

Ultrasonic Testing


These studies need the highest number of signals as possible.

Laboratory tests must be accurate and repeated several times to obtain reliable signals.

↓

In this phase of the work becomes important to simulate effectively the real models with a suitable FEM code.

Ultrasonic Testing


The Ultrasonic Technique has been carried out on a trachyte stone masonry

with a cavity inside.

Front view of the wall

Horizontal plane section

Experimental session

Vertical plane section


The wall is 90 cm wide, 62 cm high and 38 cm thick, and it is made of trachyte blocks sized 20 × 38 × 12 cm3 joined with mortar.

To map the faults

308 points for the emitter transducer and 308 points for the receiver transducer

have been arranged in a grid of 14 x 22 nodes in the

opposite surfaces of the wall.



Ultrasonic measurements have been carried out using the ultrasonic test equipment Pundit Lab+, developed by

Proceq, with standard 54 KHz transducers .

Experimental set-up


The energizing signal is a square wave with high input voltage of 500 V, which develops in a time interval of 9.3μs.

The high intensity of this signal allowed signals on the other side of the wall to be detected even if strongly attenuated.

Vo

ltag

e [

V]



A signal has been acquired in each point of the grid of receivers. From each node of the grid, several features in both time and frequency domain can been extracted.


Time domain • Propagation velocity • Maximum peak-peak amplitude • Maximum absolute amplitude • Minimum absolute amplitude • Mean value of normalized amplitude • Variance of normalized amplitude • Time–reversibility • Third order auto-covariance • Mean value of normalized signal envelope • Variance of normalized signal envelope • Local rise time from 25% level to peak of

normalized signal envelope • Local rise time from 50% level to peak of

normalized signal envelope • Signal power • Travel time

Frequency Domain • Spectrum principal frequency

• Spectrum normalized maximum amplitude

• Input signal attenuation

• Spectrum central frequency

• Spectrum Bandwidth

• Spectrum normalized mean value

• Spectrum normalized variance

Some of the features that are normally considered to extract information about faults from the signal



It is possible to distinguish different types of signal paths, corresponding to different

materials regions in the wall :

trachyte path (pale blue) 2

trachyte-mortar path (yellow) 1 and 3

trachyte-mortar-air path (brown) 4

Received signals have different patterns depending on the nature of the materials crossed.

Map associated to the different materials which make up the wall.


2

3 4

To different paths correspond different signals with unlike

amplitudes and

harmonic content.

Due to edge effects, signals travelling

close to the boundary of a material region are different from those crossing the inner part of the

same region.

1

Experimental signals crossing the 4 paths (Time domain)


The harmonic content of the

signals takes place around the

resonant frequency of the

transducer (54 KHz) and differs for each

path.

1

3

2

4

Spectra (Frequency

domain)


The numerical analyses have performed using a Pressure Acoustics model and the Piezoelectric Devices Interface belonging to the COMSOL Acoustics Module. In order to decrease the size of the resolution matrices, with significant reduction of processing time and computational errors :

the frequency domain rather than the time domain has been chosen to solve the model. The sound field is described and solved by the pressure p, so we can simulate only one transducer (the emitter)

two sub-models coupled using the Extrusion Model Couplings option of COMSOL have been considered:

one for the emission transducer (2D axisymmetric) and another for the wall (3D).

Simulations with COMSOL Multiphisycs


The wall and the emitter transducer have been simulated with two submodels




Two sub-models coupled

Emission transducer model (2D axisymmetric) Wall model (3D)


The sound field is described and solved by the pressure p. The acoustic pressure in the time domain p(t) is obtained as a function of the spectral components as:


maxmin

1

2cos

fff

fptffptp

i

N

iiphaseiamp

N number of components of the spectra in the frequency range


The acoustic pressure p(t ) in a medium is governed by the following equation that is an inhomogeneous Helmholtz equation:

2

22

2

; ;

1

c

ceq

bt

c

t

eqt

c

c

cj

ck

ck

ppp

Qp

kp

• ρ is the density of the material [kg/m3], • c is the speed of sound [m/s]; • Q [1/s2] is a monopole acoustic source


Analytic model of Pressure Acoustics, Frequency Domain interface


The mathematical formulation of this model is described by the equations: where • Fv is deformation gradient, • ρV is the volume charge density, • u is the displacement, • D is the electric displacement and • σ is the stress-charge.

V

jv

D

eFu

2


Analytic model of Piezoelectric Devices interface


napn

0

1

At the interface between the transducer and the wall, the boundary condition for the acoustics interface is that the pressure is equal to the normal acceleration of the solid domain:


where an is the normal acceleration. This drives the pressure in the wall domain.



The implementation of the Comsol Model has required a series of simulations to develop all the parameters, until than the results were consistent with the experimental ones.


Simulated signals show a

performance in good agreement

with experimental signals

Amplitudes in the time domain are

scaled to the average gain of the

emission transducer

2

3 4

1

Simulated signals crossing the 4 paths (Time domain)


Spectral bandwidth are shifted to

higher values , as expected

1

3

2

4

Simulated Spectra (Frequency domain)


Results

Similarly to what is found for the measured signals:

also the simulated signals present different performances for the different types of paths both in time and frequency domain, and

this allows to post-process them to map the macro defects.


Conclusions

The purpose of this work was to create a model of a prototype of a trachyte wall, built and tested in the laboratory, in order to simulate the ultrasonic signal propagation and thus be able to perform parametric studies freed from the experimental sessions.

Model calibration has been achieved by comparison of a significant sample of simulated signal to the correspondent signals obtained from the experimental sessions

Results show that the implemented COMSOL model is suitable to effectively simulate the ultrasonic signals transmission through the stone wall

The simulated signals can be used to obtain, through post-processing analysis, maps for detecting the presence of macro defects with cluster algorithms or non supervised Neural Networks as Self organizing Maps (SOM)


Thanks for your kind attention

For further information:

[email protected]

[email protected]




Cagliari

calibration of ultrasonic testing for faults detection in ... · calibration of ultrasonic testing...

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